Formation, evolution and vanishing of bubbles are common phenomena in nature, which can be easily observed in boiling or falling water, carbonated drinks, gas-forming electrochemical reactions and so on. However, the morphology and the growth dynamics of the bubbles at nanoscale have not been fully investigated owing to the lack of proper imaging tools that can visualize nanoscale objects in the liquid phase. Here, we demonstrate for the first time that the nanobubbles in water encapsulated by graphene membrane can be visualized by in-situ ultra-high vacuum transmission electron microscopy. Our microscopic results indicate two distinct growth mechanisms of merging nanobubbles and the existence of a critical radius of nanobubbles that determines the unusually long stability of nanobubbles. Interestingly, the gas transport through ultrathin water membranes at nanobubble interface is free from dissolution, which is clearly different from conventional gas transport that includes condensation, transmission and evaporation.

The surface-enhanced Raman scattering (SERS) of 4-aminobenzenehtiol (4-ABT) has seen a surge of interest recently, since its SERS spectral features are dependent not only on the kinds of SERS substrates but also on the measurement conditions. The most unusual SERS feature is the appearance of b 2-type bands in the region 1100–1500 cm–1, in contrast to their absence in the normal Raman spectrum, but their origin is not yet clarified. However, propositions have been made suggesting that their appearance is associated with either a charge transfer phenomenon or a surface-induced photoreaction product such as 4,4′-dimercaptoazobenzene (4,4′-DMAB). In this work, we found that the b 2-type bands of 4-ABT are strongly affected also by the solution pH. Regardless of the excitation wavelength and kind of SERS substrates, the b 2-type bands appeared very weakly or negligibly at acidic pHs, while they were observed very distinctly at basic pHs. For the case of 4,4′-DMAB, any such pH dependence was not observed at all in its SERS spectra. Since the pH dependence in the SERS of 4-ABT was observed reversibly, the appearance and disappearance of the b 2-type bands must have nothing to do with formation of any surface-induced photoreaction product like 4,4′-DMAB. Consulting the pH-dependent UV–vis absorption spectra and ab initio quantum mechanical calculation, the disappearance of the b 2-type bands at acidic pHs is presumed to be associated with the upshift of the lowest unoccupied molecular orbital level of 4-ABT caused by protonation of the amine group: the charge transfer resonance chemical enhancement will then be less likely to occur.

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Raman scattering measurements were conducted for 4-aminobenzenethiol (4-ABT) monolayers assembled on a macroscopically smooth Pt substrate. At the beginning, no Raman peak was detected for 4-ABT on Pt, but upon attaching Ag nanoparticles to the amine groups of 4-ABT on Pt (Ag@4-ABT/Pt), distinct Raman spectra were observed. Considering the fact that almost no Raman peaks are observed when Ag nanoparticles are attached to 4-aminophenylsilane monolayers assembled on a silicon wafer, the Raman spectra observed for Ag@4-ABT/Pt must be surface-enhanced Raman scattering (SERS) spectra, occurring through an electromagnetic (EM) coupling of the localized surface plasmon of Ag nanoparticles with the surface plasmon polariton of the Pt substrate. From the excitation wavelength dependence, we also confirmed the contribution of the charge-transfer enhancement in the SERS spectra of Ag@4-ABT/Pt: it became more important at short-wavelength excitation. Overall, the SERS intensity of Ag@4-ABT/Pt gradually decreased as the excitation wavelength was increased from 488 to 514.5, 568, and 632.8 nm. A similar trend was observed with a finite-difference time-domain calculation, suggesting that the EM coupling should also be strong at short-wavelength excitation. Accordingly, the experimental enhancement factor per Ag nanoparticle was estimated to be as large as 7.9 × 102 under the illumination of 514.5 nm radiation. The present observation clearly demonstrates that the inherent obstacles to the more widespread use of SERS can be overcome by the judicious use of SERS-active nanoparticles directly or indirectly.

The surface-enhanced Raman scattering (SERS) of 4,4'-dimercaptoazobenzene (4,4'-DMAB), an alpha, omega-dithiol possessing also an azo moiety, has seen a surge of interest recently, since 4,4'-DMAB might be able to form from 4-aminobenzenethiol (4-ABT) via a surface-induced photoreaction. An understanding of the intrinsic SERS characteristics of 4,4'-DMAB is thus very important to evaluate the possibility of such a photoreaction. We found in this work that 4,4'-DMAB should adsorb on a flame-annealed Au substrate via one of its two thiol groups such that Au nanoparticles could adsorb further on the pendent thiol group, forming a SERS hot site. The most distinctive feature in the SERS of 4,4'-DMAB was the appearance of a(g) bands, which were quite similar to the b(2)-type bands occurring in the SERS of 4-ABT. In an electrochemical environment, the a(g) bands of 4,4'-DMAB at 1431, 1387, and 1138 cm(-1) became weakened at lower potentials, completely disappearing at -1.0 V, but the bands were restored upon increasing the electrode potential, implying that neither electro- nor photo-chemical reaction to break the azo group took place, in agreement with data from a cyclic voltammogram. The appearance and disappearance of these a(g) bands are thus concluded to be associated with the charge transfer phenomenon: 4,4'-DMAB must then be one of a unique group of compounds exhibiting chemical enhancement when subjected to a SERS environment.

Some threshold energy is required for 4-aminobenzenethiol to occupy the gold surface sites capable of leading to the appearance of surface-enhanced Raman scattering peaks via a charge transfer resonance enhancement mechanism.

4-Aminobenzenthiol (4-ABT) is an unusual molecule, showing variable surface-enhanced Raman scattering (SERS) spectra depending upon measurement conditions. In an effort to reduce ambiguity and add clarity, we have thus conducted an ultraviolet-visible (UV-vis) extinction measurement, along with Raman scattering measurement, after adding 4-ABT into aqueous Ag sol. Upon the addition of 4-ABT, the surface plasmon absorption band of Ag at 410 nm gradually diminished and, concomitantly, a weak and broad band developed at longer wavelengths, obviously because of the aggregation of Ag nanoparticles. At the same time, the Raman scattering peaks of 4-ABT varied in intensity as the Ag particles proceeded to form aggregates. A close examination revealed that the peak intensity of the ring 7a band of 4-ABT, a typical a(1) vibrational mode, could be correlated with the UV-vis extinction of the Ag sol measured at the excitation laser wavelength. In a separate Raman measurement conducted using sedimented Ag colloidal particles, 4-ABT was found not to be subjected to any surface-induced photoreaction, implying that all of the observable Raman peaks were, in fact, solely due to 4-ABT on Ag. The intensities of the b(2)-type bands, such as the ring 3, 9b, and 19b modes of 4-ABT, were then analyzed and found to be invariant with respect to the 7a band, irrespective of the extent of Ag aggregation as far as at a fixed excitation wavelength. The intensity ratio of the b(2)-type/7a bands would then reflect the extent of the chemical enhancement that was involved in the SERS of 4-ABT in aggregated Ag sol.

In order to resolve the dispute on the origin of the b 2 -type bands in the surface-enhanced Raman scattering (SERS) of 4-aminobenzenethiol (4-ABT), we have measured its SERS spectra under a variety of conditions, including variable temperature and rotation, electrochemistry, and pH, as well as in the presence of a reducing agent. For comparison, the SERS spectra of 4-nitrobenzenethiol (4-NBT) and methyl orange (MO), a prototype azo compound, were also measured. First, we found that 4-ABT on Ag is not subjected to photoreaction, although 4-NBT is highly photoreactive on a silver surface. In the electrochemical environment, b 2 -type bands of 4-ABT lost their intensity at very negative potentials, but the intensity recovered immediately upon raising the potential. In addition, b 2 -type bands were observed under rotation even after lowering the potential. The disappearance and reappearance of the b 2 -type bands could also be observed by bringing the sample of 4-ABT on Ag into contact consecutively with a borohydride solution and water. This is because the surface potential of Ag is lowered by contact with a borohydride solution. Besides, we found that not only the normal Raman but also the SERS spectral features of 4-ABT are hardly affected by pH variation, while the spectral features of MO are greatly affected, especially in the region of the N N stretching vibration, suggesting that the possibility of a photoconversion of 4-ABT to an azo compound is low. Altogether, the b 2 -type bands were attributed to 4-ABT, appearing in conjunction with the chemical enhancement mechanism in SERS.